Mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, and other electronic devices utilize light-emitting diodes (“LEDs”), organic light-emitting diodes (“OLEDs”), polymer light-emitting diodes (“PLEDs”), and other SST devices for backlighting. SST devices are also used for signage, indoor lighting, outdoor lighting, and other types of general illumination. FIG. 1A is a cross-sectional view of a conventional LED device 10a with lateral contacts. As shown in FIG. 1A, the LED device 10a includes a substrate 20 carrying an LED structure 11 having an active region 14, e.g., containing gallium nitride/indium gallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”), positioned between N-type GaN 15 and P-type GaN 16. The LED device 10a also includes a first contact 17 on the front surface of the P-type GaN 16 and a second contact 19 spaced laterally apart from the first contact 17 on the front surface of the N-type GaN 15. The first contact 17 typically includes a transparent and conductive material (e.g., indium tin oxide (“ITO”)) to allow light to escape from the LED structure 11.
FIG. 1B is a cross-sectional view of another conventional LED device 10b in which the first and second contacts 17 and 19 are opposite each other, e.g., in a vertical rather than lateral configuration. During formation of the LED device 10b, the N-type GaN 15, the active region 14 and the P-type GaN 16 are stacked sequentially on a growth substrate (not shown), similar to the substrate 20 shown in FIG. 1A. The first contact 17 is formed on the P-type GaN 16, and a carrier substrate 21 is attached to the first contact 17. The growth substrate is then removed and the second contact 19 is formed on the N-type GaN 15. The structure is then inverted to produce the orientation shown in FIG. 1B. The N-type GaN 15 at the front surface of the LED device 10b provides better current spreading than the P-type GaN 16. The vertical LED device 10b also has enhanced light extraction and thermal properties, and accordingly a higher efficiency than the lateral LED device 10a of FIG. 1A.
Typical LEDs have relatively low forward junction voltages (or built-in voltages) compared to the power supplies with which they are used. For example, gallium nitride/indium gallium nitride (GaN/InGaN) based LED dies typically operate at a forward junction voltage of approximately 3 volts direct current (“DC”) and aluminum indium gallium phosphide (AlInGaP) based LED dies typically have a forward junction voltage of approximately 2 volts DC, whereas many power supplies operate at 48 volts alternating current (“AC”), 120 volts AC, 60 volts DC, etc. Therefore, power supplies typically include AC/DC rectifiers, DC/DC converters, power conditioners, drivers, and/or other suitable components to supply power to LED dies at suitable voltage levels. However, power supplies and associated components operate more efficiently when the difference between the output voltage and the input voltage is smaller. Accordingly, high-voltage LEDs (e.g., 24 volts, 60 volts, etc.) are preferred for use with high-voltage power supplies to enhance the overall efficiency of the LED system.
Conventional high-voltage LEDs are made by coupling several lateral LED dies (e.g., the lateral LED device 10a of FIG. 1A) together in series. For example, twenty lateral LED dies, each having a forward junction voltage of 3 volts, can be serially coupled to operate at a combined forward junction voltage of 60 volts. Lateral LEDs, however, have several performance limitations. For example, referring to FIG. 1A, the P-type GaN 16 at the front surface of the lateral LED device 10a does not inherently provide current spreading, and therefore current and light concentrate under the first contact 17. To increase current spreading across the lateral LED device 10a, the first contact 17 must be thickened and extend over a larger portion of the P-type GaN 16, which makes the first contact 17 less transparent and decreases the light extraction from the LED device 10a. Additionally, lateral LEDs typically have poor thermal characteristics and low overall efficiency. Accordingly, there is a need for high-voltage LEDs and other high-voltage SSTs with enhanced efficiency and performance.